1. Field of the Invention
This invention relates to a nitrogen-containing dual phase stainless steel with excellent hot workability.
2. Discussion of the Background
In applications where resistance to stress corrosion cracking is an important factor, for example, in the case of heat exchangers to be used in chemical industries, it has been the conventional practice to employ an ASME SA 669 steel which is a typical dual phase stainless steel. However, a steel of this sort has a problem in that its ferrite content is increased at the time of welding by application of high heat temperature deteriorating the corrosion resistance of the weld including the welding-heat affected zones by precipitating Cr carbides in ferrite grain boundaries upon cooling in a subsequent stage.
In an attempt to prevent deteriorations of the corrosion resistance of the weld, it has been proposed (in Japanese Patent Publication No. 59-5662) to produce a nitrogen-containing dual phase stainless steel in which an austenite phase is reserved in the high-temperature affected zones by addition of N which is an austenite forming element. However, since N is solid-soluble mainly in the austenite phase, this N-containing dual phase stainless steel also has a problem in that cracking is apt to occur at the boundaries of the austenite and ferrite phases due to a large difference in hot deformation resistance therebetween, resulting in inferior hot workability.
In the forging and rolling methods which are generally resorted to for cogging steel ingots, it is essential to employ a rolling method from a practical and economical point of view. Accordingly, the present inventors conducted an extensive study with a view to improving hot workability of nitrogen-containing dual phase stainless steel, and as a result, found that a N-containing dual phase stainless steel capable of blooming and retaining sufficient resistance to stress corrosion cracking could be obtained by restricting the S and Ca contents of the steel in a certain correlation with each other.